Stiff cured rubber compositions are often used for various tire components. For example, various rubber compositions are often used as internal sidewall components for promoting stiffness for tire sidewalls to enhance tire handling and cornering stability.
For this invention, the cured rubber component stiffness is promoted by an inclusion of a reinforcing resin as well as one or more relatively high viscosity elastomers in the rubber composition.
A problem with providing such stiff cured rubber composition is that, in its uncured state, it is relatively difficult to process such as, for example, by extrusion, particularly where a reinforcing, rubber stiffness promoting, resin is mixed with the already viscous uncured rubber composition.
Accordingly, it is desired to evaluate whether an improvement of the processing of the uncured rubber composition may be accomplished.
Historically, 4-hydroxyTEMPO, namely 2,2,6,6-tetramethylpiperidine-1,4-diol, has been used as a free radical scavenger and in various rubber compositions. For example, see U.S. Pat. Nos. 3,334,103; 6,084,015; and 6,194,509; U.S. Patent Application Nos. 2008/0051496, 2008/0085973, and 2009/0069469 as well as literature references “Mechanisms of Antioxidant Action”, L. P. Nethsinghe, et al, University of Aston in Birmingham, England, received Jan. 15, 1984, (Rubber Chemistry and Technology, Volume 57, Pages 779 through 791); “Mechanisms of Antioxidant Action”, H. S. Dweik, et al, University of Aston in Birmingham, England, received Dec. 2, 1983, (Rubber Chemistry and Technology, Volume 57, Pages 908 through 917); “Mechanisms of Antioxidant Action” H. S. Scott, et al, received Apr. 20, 1983 (Rubber Chemistry and Technology, Volume 57, Pages 735 through 743).
For this evaluation, the 4-hydroxy TEMPO is blended with the rubber composition prior to addition of the reinforcing resin.
Further, for this evaluation, it is required that the elastomer(s) of the rubber composition be primarily composed of isoprene-containing elastomers in a sense that the isoprene repeat unit is in the backbone of the elastomer either as a homopolymer of isoprene or as a copolymer of isoprene with other monomer comprised of, for example, at least one of butadiene and styrene.
As will be observed, it was discovered that blending of the 4-hydroxyTEMPO in the rubber composition followed by a subsequent blending of the reinforcing resin enabled a rubber composition containing a diene-based elastomer, particularly an isoprene-containing elastomer, which has a unique combination of low processing viscosity for the uncured rubber composition and a high cured stiffness for the sulfur vulcanized rubber composition.
While the mechanism may not be entirely understood, it appears that the radicals formed during high shear mixing of the rubber composition due to polymer chain scission in the isoprene-containing elastomer have an ability to combine with the stable free radical that is present in the 4-hydroxyTEMPO. It is known that the radical formed at the isoprene repeat unit in the elastomer during the high shear mixing of the rubber composition is longer lived (has a longer life) than similar radicals formed for butadiene repeat units. It is therefore envisioned that the combination of the isoprene radical and 4-hydroxyTEMPO radical results in a stabilization of the shorter elastomer chains which had been formed during the high shear mixing process due to chain scission.
However, it has been observed that when the reinforcing resin, namely a phenol formaldehyde resin, is added together with the 4-hydroxyTEMPO, instead of subsequent to the 4-hydroxyTEMPO addition, to the rubber composition containing the isoprene-containing elastomer for the high shear rubber mixing, the resulting uncured rubber composition still exhibited a high viscosity similar to the relatively high viscosity caused by the introduction of the phenol formaldehyde resin without the 4-hydroxyTEMPO, indicating that the presence of the 4-hydroxyTEMPO made little difference in the Mooney viscosity of the phenol formaldehyde-containing rubber composition. This continued high viscosity of the rubber composition may be due to the phenolic groups of the phenol formaldehyde combining with the stable free radical of the 4-hydroxyTEMPO, thus disabling their ability to combine with and stabilize the isoprene free radicals formed by the chain scission during the high shear mixing.
It is considered that this is the essence of, and a basis for, the significant discovery for this invention.
For this invention, then, it is proposed to provide a more easily processable uncured rubber composition (e.g. more easily extruded) which can provide a relatively stiff cured rubber composition, for example, for tire components such as, for example, tire sidewall stiffening components. Such sidewall components may be, for example, a sidewall apex or a tire sidewall insert.
Accordingly, an evaluation is proposed for an inclusion of a free radical absorbing compound such as, for example, 4-hydroxytempo (or, 2,2,6,6-tetramethylpiperidine-1,4-diol) to stabilize free radicals formed during by cleavage of portions of a diene-based elastomer during high shear mixing of an uncured diene-based elastomer in a rubber composition which contains reinforcing filler selected from at least one of rubber reinforcing carbon black and amorphous silica such as, for example precipitated silica.
For this evaluation, the elastomers are isoprene-containing elastomers because, as earlier discussed, the radicals formed on the isoprene repeat units during the high shear mixing of the rubber composition have a longer lifetime than radicals formed from butadiene or styrene units. This longer lifetime is envisioned to allow more time for the radicals of the isoprene unit and the stable free radical 4-hydroxyTEMPO to beneficially combine to promote a reduction in the Mooney viscosity of the rubber composition.
Representative of such isoprene-containing elastomers are, for example, cis 1,4-polyisoprene rubber (natural and synthetic), isoprene/butadiene copolymer elastomers, styrene/isoprene/butadiene terpolymer elastomers and 3,4-polyisoprene elastomers.
The rubber composition with such stabilized created free radicals is intended to promote a lower rubber viscosity for the uncured rubber having been mixed under free radical generating high shear conditions to therefore the uncured rubber more easily processable such as by extrusion of the rubber composition.
It is then proposed to evaluate an addition of one or more phenol formaldehyde resins to such uncured rubber composition, subsequent to such free radical generation 4-hydroxytempo stabilization treatment, to promote stiffness for the ultimately cured rubber composition such as, for example, a phenol formaldehyde resin, particularly a pre-formed phenol formaldehyde resin, for which the aforesaid lower viscosity uncured 4-hydroxytempo treated rubber composition may be able to tolerate such resin addition in a sense that the uncured rubber composition might be of a suitable viscosity that it is easily processable such as by extrusion.
Accordingly, it is an aspect of this invention to undertake such evaluation.
In the description of this invention, the terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “rubber composition”, “compounded rubber” and “rubber compound”, may be used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise indicated. The term “Tg”, if used, means the middle point glass transition temperature of an elastomer determined by DSC (differential scanning calorimeter) at a heating rate of 10° C. per minute as would be understood by those having skill in such art.